Hierarchical Fe2O3 hexagonal nanoplatelets anchored on SnO2 nanofibers for high-performance asymmetric supercapacitor device

Metal oxide heterostructures have gained huge attention in the energy storage applications due to their outstanding properties compared to pristine metal oxides. Herein, magnetic Fe2O3@SnO2 heterostructures were synthesized by the sol–gel electrospinning method at calcination temperatures of 450 and 600 °C. XRD line profile analysis indicated that fraction of tetragonal tin oxide phase compared to rhombohedral hematite was enhanced by increasing calcination temperature. FESEM images revealed that hexagonal nanoplatelets of Fe2O3 were hierarchically anchored on SnO2 hollow nanofibers. Optical band gap of heterogeneous structures was increased from 2.06 to 2.40 eV by calcination process. Vibrating sample magnetometer analysis demonstrated that increasing calcination temperature of the samples reduces saturation magnetization from 2.32 to 0.92 emu g-1. The Fe2O3@SnO2-450 and Fe2O3@SnO2-600 nanofibers as active materials coated onto Ni foams (NF) and their electrochemical performance were evaluated in three and two-electrode configurations in 3 M KOH electrolyte solution. Fe2O3@SnO2-600/NF electrode exhibits a high specific capacitance of 562.3 F g-1 at a current density of 1 A g-1 and good cycling stability with 92.8% capacitance retention at a high current density of 10 A g-1 after 3000 cycles in three-electrode system. The assembled Fe2O3@SnO2-600//activated carbon asymmetric supercapacitor device delivers a maximum energy density of 50.2 Wh kg-1 at a power density of 650 W kg-1. The results display that the Fe2O3@SnO2-600 can be a promising electrode material in supercapacitor applications.

Preparation of nanofibers. The Fe 2 O 3 @SnO 2 composite nanofibers were prepared by sol-gel electrospinning method at different calcination temperatures. Firstly, 0.81 g FeCl 3 ·6H 2 O and 0.34 g SnCl 2 ·2H 2 O were stirred in 7.5 ml of ethanol and DMF as solvents for 30 min. Then, 0.6 g PVP was gradually added to the mixture and magnetically stirred overnight. The obtained sol was transferred into 10 ml syringe with a needle of 23G which was fed by a syringe pump at the rate of 0.4 ml h -1 and a high voltage of 16.5 kV to produce nanofiber composites. The needle was set at a distance of 10 cm from the aluminum foil collector. The as-spun nanofibers were obtained by drying at 100 °C for 18 h in the oven. Finally, Fe 2 O 3 @SnO 2 -450 and Fe 2 O 3 @SnO 2 -600 composite nanofibers were generated at the calcination temperatures of 450 and 600 °C for 2 h with a ramp rate of 2 °C min -1 under an air furnace, respectively. The synthesis schematic is displayed in Fig. 1. Characterization. The thermal properties of the as-spun Fe 2 O 3 @SnO 2 composite were performed by thermogravimetric-derivative thermogravimetry (TGA-DTG) analysis using Bahr model STA 504 thermal analyzer instrument. The structure of the prepared samples was analyzed by X-ray diffraction (X'Pert Pro, Panalytical) using Cu Kα (λ = 1.5406 Å) radiation. Fourier-transform infrared (FTIR) spectra of the samples were recorded by the Bruker Alpha in the range of 400-4000 cm -1 . Field emission scanning electron microscopy (FESEM) with an energy dispersive spectroscopy (EDS, MIRA3, TESCAN-XMU) was used to investigate the morphology of the mentioned samples. The absorption spectra of the samples were measured by Varian Cary 100 UV/Visible spectrophotometer. The magnetic parameters of the composites were investigated using a vibrating sample magnetometer (VSM, Magnetic Daghigh Kavir Co., Iran) at room temperature (300 K) with a maximum applied magnetic field of ± 15 kOe.
Electrochemical measurements. The working electrodes were prepared by mixing active materials (Fe 2 O 3 @SnO 2 composites), carbon black and PVDF as a binder at a weight ratio of 80:10:10 in NMP as a solvent to form a homogeneous slurry. The Ni foam (NF) substrates (2 × 1 cm 2 ) were successively cleaned in aqueous HCl (3 M), deionized water, ethanol, and acetone for removing the NiO layer using an ultrasonic device each for 30 min, subsequently dried in the oven at 65  www.nature.com/scientificreports/ 1 mg. The electrochemical measurements of the electrodes were executed in a three-electrode system by Zahner Zennium device, comprised of Fe 2 O 3 @SnO 2 composite as a working electrode, platinum wire as a counter electrode and Ag/AgCl as a reference electrode in 3 M KOH electrolyte at room temperature. Cyclic voltammetry (CV) was recorded at a potential window from 0 to 0.5 V, galvanostatic charge-discharge (GCD) was tested in the potential between 0 and 0.43 V, and the electrochemical impedance spectroscopy (EIS) was performed at the frequency ranges from 100 kHz to 10 mHz at an open circuit potential of 0.01 V and 5 mV AC amplitude. The impedance spectra were fitted by the equivalent circuit using the Z-view software.

Result and discussion
Thermal analysis. The suitable temperature to decompose PVP, remove of residual compounds and form Fe 2 O 3 @SnO 2 nanofibers was determined through the thermogravimetric measurement in argon atmosphere from room temperature to 800 °C with a heating rate of 10 °C/min. The TGA and its derivative (DTG) curves for the as-spun nanofibers are shown in Fig. 2, demonstrating the weight loss percentage as a function of temperature. Three distinct stages were observed in weight loss. The first stage with a weight loss of about 12.53% at the range of room temperature to 200 °C could be ascribed to the evaporation of the solvents, including ethanol and DMF in the as-spun nanofibers 35 . The second weight loss in the temperature range of 200-350 °C corresponds to the decomposition of the metal precursors, which is verified by an endothermic peak centered at 240 °C in    In the case of non-metallic bonds, the one that can be observed at 3427 cm -1 corresponds to OH stretching vibration of water molecules. The bands which are located around ~ 2960 and 2929 cm -1 can be related to asymmetric stretching vibration of CH 2 and the one that is located around 2864 cm -1 can be attributed to symmetric stretching vibration of CH 2 bond. The two absorptions are seen at 1728 and 1632 cm -1 which are assigned to stretch vibrated C = O and that one is observed at 1281 cm -1 can be related to asymmetric stretching vibration of C-N. The deformed modes of CH and NCH bonds can be identified around 1462 and 1384 cm -1 , respectively. Finally,  www.nature.com/scientificreports/ the absorption located at 1127 and 1073 cm -1 can be assigned to rocking vibration and at 840 cm -1 is ascribed to bending vibration of C-H [40][41][42] . The absorption intensity of non-metallic bands dramatically decreased with increasing temperature up to 600 °C which can be in agreement with the fact that polymeric compounds such as PVP and DMF almost eliminated at high temperatures.

Morphological properties.
The FESEM images of the as-spun and calcined Fe 2 O 3 @SnO 2 nanofibers at different temperatures are revealed in Fig. 5. The as-spun nanofibers ( Fig. 5a-c) a smooth and uniform surface with an average diameter of about 349 nm. After the nanofibers were annealed at 450 °C, the average diameter of nanofibers shrunk to 189 nm because of the decomposition of the polymer. The average diameter promotes to 303 nm when the calcined temperature reaches to 600 °C. This phenomenon could be ascribed to the particle growth of the metal oxides 35 . As shown in Fig. 5d-i, the hexagonal plates have grown hierarchically on the hollow nanofibers, which reduce with increasing calcination temperatures from 450 to 600 °C. The Hollow interiors for nanofibers are distinguished in Fig. 5d, g, which is clearly demonstrated that open-ended nanotubes could be maintained during calcination. These open tubular architectures will largely facilitate the ion migration between active material on the electrode surface and electrolyte in the electrochemical process 43 . EDS map-scan sum spectra in Fig. 5c, f, i, revealed the presence of C, O, Fe and Sn elements, which carbon was eliminated with increasing calcination temperature up to 600 °C due to complete thermal decomposition of PVP and metal oxidation.
According to the XRD results, the reduction of the hexagonal platelets on the nanofibers by increasing calcination temperature can be associated with a decrease in the fraction of Fe 2 O 3 phase, For further clarification of elemental distribution, EDS point-scan analysis of Fe 2 O 3 @SnO 2 -600 composite were recorded in two regions (A and B), which are marked in the FESEM image (Fig. 6). The EDS spectra indicate that Sn and O elements are the dominant constituent elements of nanofibers (Region A) while the content of Fe and O is superior for the hexagonal platelet (Region B). The results prove the formation of Fe 2 O 3 hexagonal platelets anchored on SnO 2 nanofibers. Similar morphology was observed for selenization of electrospun carbon nanofibers, including tris(acetylacetonate) iron (III) with polyacrylonitrile (PAN) polymer, at different temperatures under H 2 Se gas, which resulted in the decoration of FeSe nanocrystals on the carbon nanofiber surfaces 44 . Optical band gap. UV-Vis absorption analysis was carried out to study of optical properties of the products. Tauc plot was employed to estimate the optical band gap of nanofibers. The optical absorption spectra using Tauc's relation 45 : where A * is a constant, α is the absorption coefficient, and hν is the photon energy. The absorption coefficient, α was determined from absorption data using the relation 46 : where d is the sample thickness which is about equivalent to the quartz cell's path length, and A = ln I 0 I t is the absorbance. The variation of (αhν) 2 vs. photon energy (hν) for relevant composite is shown in the inset of Fig. 7. The direct optical band gap of nanocomposites was calculated via extrapolating the linear part of the (αhν) 2 versus (hν) curve to intercept the energy axis (αhν = 0). According to the results by increasing temperature up to 600 °C, the E g value was increased from 2.06 to 2.40 eV. This band gap widening can be related to the enhancement of SnO 2 fraction in composite with an increase in the calcination temperature. The hollow Fe 2 O 3 @SnO 2 -600  Electrochemical properties. To investigate the electrochemical performance of prepared electrodes, the  www.nature.com/scientificreports/ scan rates, which implies outstanding electrochemical performance 53 . Furthermore, the pair of redox peaks are nearly symmetrical, which means the high reversibility of the electrodes. The possible redox reactions for Fe 2 O 3 and SnO 2 could be explained by the following equations 54,55 : The specific capacitance (C s ) for both electrodes from CV curves was calculated by the following Eq. 56 :  The GCD curves of both electrodes in the potential range between 0 and 0.43 V at different current densities are depicted in Fig. 10a,b. To avoid the water electrolysis (oxygen evolution reaction) during charging process, a smaller potential window than the CV curve was chosen 58,59 . The pseudocapacitive behavior of the two electrodes was confirmed from potential plateaus in the GCD curves, which correspond to the CV curves in Fig. 9. Also, the IR drop in the GCD curves of both electrodes at the beginning of the discharge time can be due to the internal resistance and energy loss of the electrode materials. The specific capacitance (C s ) is computed from GCD curves using the following equation 56 : where m is the mass of active material on the electrode (g), I is discharge current (A), V is the potential window (V), and t is discharge time (s). The graph of the C s values at different current densities for Fe 2 O 3 @SnO 2 -(450 and 600)/NF electrodes is shown in Fig. 11a. The maximum values of C s for Fe 2 O 3 @SnO 2 -600/NF electrode were 562.3, 528.8, 508.1, 459.1 and 397.7 F g -1 at current densities of 1, 3, 5, 7 and 10 A g -1 with 70.7% capability. Also, the specific capacitances for Fe 2 O 3 @SnO 2 -450/NF electrode were 365.3, 258.8, 201.2, 174.2 and 162.8 F g -1 at the same current densities with 44.5% capability. The active sites of the electrode at low current densities can appropriately react with electrolyte ions, but at high current densities, the redox reactions only occur on the surface of  www.nature.com/scientificreports/ the active materials due to the limitation of ion diffusion which leads to a decrease of the C s values 57 . Figures 9c  and 10c are given to compare the CV (at 10 mV s -1 ) and GCD (at 1 A g -1 ) curves for the two electrodes, respectively. As shown in Fig. 11a, with increasing calcination temperature from 450 to 600 °C the specific capacitance is enhanced. It can be due to the reduction of the hematite phase and promotion of the cassiterite phase which leads to more conductivity of the electrode [60][61][62] . Moreover, Fig. 11b     www.nature.com/scientificreports/ Fig. 11c. All the spectra were fitted using the equivalent circuit (as displayed in the inset of Fig. 11c). The x-axis intercept at high frequencies, the depressed semicircle at high-medium frequencies and the linear line at lower frequencies are assigned to solution resistance (R s ), charge transfer resistance (R ct ) at the electrodes/electrolyte interface and Warburg impedance (W), respectively 6,67 . Also, the constant phase element (CPE) denotes the double-layer capacitance in simulating the behavior of imperfect dielectrics. The R ct value reduced from 10.5 to 9.18 Ω, which demonstrates an increase in the conductivity of the electrodes.
To evaluate the practical applications of the Fe 2 O 3 @SnO 2 -(450 and 600) nanofiber composites, an asymmetric supercapacitor was built with the electrodes as a cathode, activated carbon pasted on nickel foam (AC/NF) as an anode electrode, and Whatman filter paper as a separator in 3 M KOH electrolyte. Figure 12a illustrates the CV curves of the individual positive electrodes (Fe 2 O 3 @SnO 2 -450 and 600) within a potential range from 0 to 0.5 V and the single negative electrode (AC/NF) from − 1 to 0 V in a three-electrode system at a scan rate of 10 mV s -1 . To achieve the high electrochemical performance of an asymmetric supercapacitor in a two-electrode system, the charge equilibrium (Q + = Q -) is essential between the two electrodes. Therefore, the mass loading of active materials on the negative and positive electrodes can be inferred by the following equation 68 : where C + (C − ) and ΔV + (ΔV − ) are the specific capacitance and working potential window of positive (negative) electrode, respectively. The specific capacitance of positive and negative electrodes can be calculated based on the CV curves using Eq. (6). Figure 12b,c show CV curves of assembled ASCs operated in the voltage range of 0-1.6 V at different scan rates. Figure 12d compares the CV curves for the two ASC devices. The redox peaks represent the pseudocapacitance contributions from the positive electrodes. Also, this pseudocapacitance characteristic is confirmed by GCD curves at various current densities (Fig. 14a,b).
The surface and diffusion controlled charge storage processes could be identified by the power-law relationship 69 : where i(V) represents the current response at a given potential, ν is a scan rate, k 1 and k 2 are constants. The slope and intercept of the linear relationship between i(V)/ν 1/2 versus ν 1/2 give the values of k 1 and k 2 , respectively. The shaded blue regions in Fig. 13b,c indicate the surface-controlled contributions at a scan rate of 10 mV s -1 for the electrodes, which occupied 37.2 and 48.9% of the total region for the Fe 2 O 3 @SnO 2 -450 and Fe 2 O 3 @SnO 2 -600, respectively (Fig. 13d). According to Eq. (7) and Fig. 14d, the C s values of Fe 2 O 3 @SnO 2 -450//AC were obtained of 195.8, 173.1, 154.1, 127.1 and 109.7 F g -1 at 1, 3, 5, 7 and 10 A g -1 , respectively, with 56% rate capability. Also, the maximum C s values for Fe 2 O 3 @SnO 2 -600//AC were achieved as 213. 9, 191.7, 180.7, 169.3, and 157.7 F g -1 at the same current densities with 73.7% rate capability. Figure 14c compares the GCD curves for the two ASC devices.
One of the most critical metrics in asymmetric supercapacitor devices is cycling stability. As shown in Fig. 14e, the cycling stabilities of the Fe 2 O 3 @SnO 2 -(450 and 600)//AC were recorded at a current density of 10 A g -1 after 3000 cycles. The ASC devices demonstrate 72 and 85% capacitance retention for Fe 2 O 3 @SnO 2 -450//AC and Fe 2 O 3 @SnO 2 -600//AC. The energy density (E s ) and power density (P s ) are the two main and  www.nature.com/scientificreports/ comparative parameters used to describe the supercapacitor performance. The E s (Wh kg -1 ) and P s (W kg -1 ) of the Fe 2 O 3 @SnO 2 -(450 and 600)//AC asymmetric supercapacitors are calculated by GCD curves using the following equations 70 : The Ragone plot is depicted in Fig. 14f, which relates the energy and power densities of the asymmetric supercapacitors. The maximum E s of 45.95 and 50.2 Wh kg -1 are achieved at a P s of 650 W kg -1 , as well as the minimum energy densities of 25.7 and 37 Wh kg -1 are retained at a higher P s of 6500 W kg - 72 . The results obtained from Fig. 14 indicate that the Fe 2 O 3 @SnO 2 -600//AC is the most suitable option for ASC device fabrication due to its high specific capacitance and long cycling stability.
The best electrode Fe 2 O 3 @SnO 2 -600 was examined as a power source. As shown in Fig. 15, two cells of the ASC device were connected in series and were able to light up the blue light-emitting diode (LED, 20 mA, 3.6 V) for about 5 min after charging by a power supply. In addition, a mini fan (0.1 W, 3 V) can be rapidly rotated by these cells for about 20 s (see Video S1). From the results, it was revealed that the Fe 2 O 3 @SnO 2 -600//AC ASC device had an outstanding performance in storing energy.

Conclusion
The hollow Fe 2 O 3 @SnO 2 nanofiber composites were successfully synthesized by the sol-gel electrospinning process at different calcination temperatures of 450 and 600 °C. The composite structures of rhombohedral and tetragonal were confirmed for hematite and cassiterite by XRD analysis, respectively. The phase percentage of SnO 2 was increased from 47.4 to 60.5% during calcination. FESEM images showed that the hexagonal nanoplatelets of Fe 2 O 3 are hierarchically anchored on the SnO 2 hollow nanofibers, which are reduced during calcination from 450 to 600 °C and verified with XRD and EDS analyses. Increasing the cassiterite phase with calcination temperatures grew the optical band gap from 2.06 to 2.40 eV due to the nature of the SnO 2 band gap. VSM results demonstrated that a significant drop in the saturation magnetization from 2.32 to 0.92 emu g -1 during www.nature.com/scientificreports/ calcination temperatures was due to the reduction of the Fe 2 O 3 phase. The electrochemical performance of the Fe 2 O 3 @SnO 2 -450 and 600 active materials pasted on the Ni foams indicated that the prepared Fe 2 O 3 @SnO 2 -600/ NF electrode has a maximum specific capacitance of 562.3 F g -1 at a current density of 1 A g -1 , a remarkable rate capability (70.7%) and excellent retention (92.8%) after 3000 cycles. Increasing the capacitance contribution from 37.2 to 48.9% during calcination distinguishes the Fe 2 O 3 @SnO 2 -600/NF electrode from another electrode. Furthermore, the assembled Fe 2 O 3 @SnO 2 -600//AC ASC device delivers a maximum energy density of 50.2 Wh kg -1 at a power density of 650 W kg -1 . Overall, this study provides a promising strategy for the production of new hollow nanofiber electrode materials that encounter high power and energy density provisions for supercapacitor applications. The hexagonal platelets Fe 2 O 3 decorated on SnO 2 hollow nanofiber is an admirable candidate for electrode material in electrochemical energy storage devices.

Data availability
All data generated or analyzed during this study are included in this published article, and the datasets used/ or analyzed during the current study are available from the corresponding author on reasonable request.